Generally, a fluorescent lamp used as a light source in a backlight unit for a liquid crystal display is a mercury lamp (a low-pressure mercury vapor discharge lamp, to be exact) with a phosphor coated on the inner surface thereof. In terms of light-emission mechanisms, fluorescent lamps are classified into a hot-cathode fluorescent lamp (hot-cathode tube) that emits light by thermionic emission and a cold-cathode fluorescent lamp (cold-cathode tube) that emits light by secondary electron emission.
The cold-cathode fluorescent lamp has a lifetime of as much as about 50,000 hours, which is as much as five times longer than that of the hot-cathode type being about 10,000 hours. Accordingly, the cold-cathode fluorescent lamp is normally used as a light source for a liquid crystal display.
In the meantime, in a large-sized liquid crystal display exceeding 30 inches, as many as 10 or more cold-cathode tubes are required due to an increase in required quantity of light following the increase in size and, therefore, there has arisen a problem that the power consumption increases to 200 W or more. Further, not only the number of cold-cathode tubes simply increases, but also an inverter is required per cold-cathode tube and, therefore, an increase in cost has also arisen as a problem. Although a measure may be considered that increases the light-emission amount from each cold-cathode tube by thickening it so as to reduce the number used per display, the light-emission efficiency of a cold-cathode tube is inversely proportional to the tube diameter, thus resulting in an increase in power consumption, and therefore, the measure using thick cold-cathode tubes is ultimately not effective. Consequently, in practice, use is made of ten or more relatively thin cold-cathode tubes having a diameter of about 2 to 3 mm.
In contrast thereto, it has also been proposed to use, instead of the cold-cathode fluorescent lamp, a hot-cathode fluorescent lamp as a light source of a backlight unit. The hot-cathode fluorescent lamp can achieve a light-emission efficiency as high as twice or more that of the cold-cathode type and further is not subjected to a reduction in light-emission efficiency even if it is thickened, and therefore, hot-cathode fluorescent lamps having a diameter of up to about 30 mm are used in illuminators and so on. Further, the light-emission amount (total luminous flux) of a single hot-cathode fluorescent lamp is normally as much as 2000 lumens or more and thus can achieve a quantity of light as much as about ten times that of a single cold-cathode tube (diameter 2 to 3 mm).
The structure of a liquid crystal display backlight unit using hot-cathode fluorescent lamps as light sources is described, for example, in Japanese Patent Application No. 2005-0076431 proposed by the present applicant. FIG. 1 shows the structure of a backlight unit similar to the proposed structure.
The structure shown in FIG. 1 comprises two hot-cathode fluorescent lamps 20a and 20b as light sources each having a shape with a longitudinal direction, i.e. a columnar shape, a rod shape, or a tubular shape, and disposed on the outside of upper and lower sides of a liquid crystal panel 10 having a rectangular plate shape, with their longitudinal direction L along those sides, respectively, and a reflector 50 serving to reflect light, radiated from the peripheries of the fluorescent lamps 20a and 20b, so as to be incident on the back surface of the liquid crystal panel 10. FIG. 1 is a cross-sectional view perpendicular to the longitudinal direction of the fluorescent lamps 20a and 20b. The reflector 50 comprises a main reflecting surface portion 51 facing the back surface of the liquid crystal panel 10, two light source back-side reflecting surface portions 52 continuous with the main reflecting surface portion 51 and facing at least regions, not facing the back surface of the liquid crystal panel 10, of the peripheries of the fluorescent lamps 20a and 20b, respectively, and a mountain-shaped portion 512 projecting in a triangular mountain shape toward the liquid crystal panel 10 at a position facing a (vertical) center portion of the liquid crystal panel 10. The mountain-shaped portion 512 is formed for obtaining a uniform light intensity distribution over the back surface of the liquid crystal panel 10. The shape of the light source back-side reflecting surface portion 52 in a cross-section perpendicular to the longitudinal direction of the fluorescent lamp is a substantially semicircular shape concentric with the periphery, circular in cross-section, of the fluorescent lamp.